1) Field of the Invention
This invention relates to a mirror that is provided in an optical switch and can reduce the diffraction influence of light reflected by the edges of its reflective surface.
2) Description of the Related Art
Conventionally, channel switching in an optical transmission system is carried out by an electrical switch after conversion of optical signals into electrical signals. However, it is possible to enhance a speed of the channel switching and its efficiency by using a switch (optical switch) that switches channels as optical signals without converting them into electrical signals.
FIG. 8A is a perspective view to represent a structure of an optical switch. FIG. 8B is a side view to represent the optical switch. An optical switch 10 is constructed from a spectral element 1 to separate wavelength-multiplexed light signals into their spectral components, an input/output optical system 2 (input optical system and output optical system) having an input port, output ports, and a collimator lens array that are arranged in an array form, and the like, a condensing optical system 3, and a movable reflector 70 provided with micro electro mechanical system (MEMS) mirrors 4 corresponding to a plurality of wavelengths that have been separated into spectral components, respectively.
The spectral element 1 shown in FIG. 8A is an example that uses transmission-type diffraction grating. This spectral element 1 is input with a wavelength multiplexed optical signals from the input port and disperses and outputs wavelength components contained in this wavelength multiplexed optical signals in different directions (horizontal direction in the drawing) for every wavelength. The movable reflector 70 is provided with a plurality of micromirrors (MEMS mirrors 4) arranged in an array form in the spectral direction (dispersion) of wavelengths by the spectral element (diffraction grafting). Among optical signals that have been separated by the spectral element 1, each MEMS mirror 4 reflects an incident optical signal to the MEMS mirror itself depending on its arrangement position, and has a function of a wavelength selection switch that leads the reflected optical signals to any one of the plural output ports in the input/output optical system 2.
Selection of output port can be carried out by changing angles of the reflective surface of each MEMS mirror 4, and independently different switching is possible with respect to a plurality of wavelengths by independently carrying out angle control of the reflective surface of each MEMS mirror 4.
For example, as shown by the dotted line in FIG. 8B, an angle of the reflective surface of one MEMS mirror 4 that constitutes the movable reflector 70 is allowed to change so as to lead reflected light to a different output port (for example, the angle is changed along the arrangement direction of the ports). Thus, it is possible to distribute a specific wavelength contained in the wavelength multiplexed optical signals input from the input port to any one of the output ports (for example, refer to Published Japanese Translations of PCT International Publication for Patent Applications No. 2003-515187).
Further, intensity of optical signals input to output ports can be attenuated not by a dynamic motion of selection of an output port but by changing slightly the angle of each MEMS mirror 4 of the movable reflector 70.
FIG. 8C is a schematic for explaining a principle of output light attenuation. An angle change of output light by changing an angle of the MEMS mirror 4 of the movable reflector 70 is converted to translation change by the spectral element 1. As shown by the solid line in FIG. 8C, when an output light beam (beam having a center wavelength) is coupled in a state where the output light beam does not deviate from the axis of the collimator lens in the input/output optical system 2, it is in a state where transmissivity is the least. However, when the angle of the reflective surface of the MEMS mirror 4 of the movable reflector 70 is slightly changed (the reflective surface of the MEMS mirror 4 is tilted further downward as shown in FIG. 8C, but it is possible for the MEMS mirror 4 to be tilted upward reversely), an axis shift between the output light beam and the collimator lens occurs as shown in FIG. 8C by the dotted line. As the magnitude of axis shift is large, the amount of attenuation increases, resulting in a decrease in intensity of the output light input to fibers of the output port.
FIG. 8D is a schematic to represent input positions of reflected beam from the movable reflector to the collimator lens. FIG. 8D represents input positions of the reflected beam from the movable reflector 70 (MEMS mirror 4) to the collimator lens arranged at the leading end of fibers of the output port.
The reflected beam from the movable reflector 70 (MEMS mirror 4) enters a spot position 21 in a state where there is no axis shift with respect to a collimator lens 20 in the input/output optical system 2 (a case of passing the path shown by the solid line in FIG. 8C). On the other hand, when there is an axis shift with respect to the collimator lens 20, the reflected beam enters a spot position 22, whereby attenuation of the reflected light input to the output fibers occurs. Such an axis shift is not limited to the case in which the angle of the MEMS mirror 4 in the perpendicular direction (vertical direction) with respect to the wavelength dispersion direction is changed in the wavelength dispersion direction (horizontal direction), but is possible by changing the angle with respect to the wavelength dispersion direction.
According to the technology explained in Background Art, attenuation control with respect to reflected light can be carried out by angle control of the reflective surface of each MEMS mirror 4; however, when practical operation is considered, it has been found that deviation of a center wavelength of optical signal irradiated on the MEMS mirror 4 should be taken into consideration.
FIG. 9A is a schematic to represent positions incident to the movable reflector. FIG. 9A represents changes in irradiation positions to the MEMS mirror 4 in a case where the center wavelength of beam incident to the MEMS mirror 4 is changed. Assuming that each beam after separating light into its components by the spectral element 1 represents the center wavelength and that each MEMS mirror 4 is set such that each beam having its center wavelength is irradiated at the center of the MEMS mirror 4, respectively, each beam should be irradiated at the center of each of the MEMS mirrors 4 as shown by 5c in FIG. 9A unless each wavelength contained in wavelength multiplexed optical signals practically deviates from its center wavelength.
However, when the center wavelength of each beam after separating light into its components deviates, each beam is irradiated at the spot shown by 5b or 5d in FIG. 9A, and when it deviates further, each beam is irradiated on the end surface sides of the MEMS mirror 4 (end surface sides of the MEMS mirror 4 with respect to the dispersion direction) such as 5a and 5e in FIG. 9A.
FIG. 9B is a schematic to represent reflected beams from the movable reflector. FIG. 9B represents how the reflected beams on the MEMS mirror 4 spread. What should be paid attention to is that spot diameters of reflected beams 6 (6a and 6e) in a case where the incident beams are irradiated to near the end surface of the MEMS mirror 4 spread at positions away from the MEMS mirror 4 compared to those of reflected beams 6 (6b to 6d) in a case where the reflected beams 6 (6b to 6d) enter near the center of the MEMS mirror 4 because part of incident beams 5 (5a and 5e) is cut off, whereby diffraction occurs.
FIG. 9C is a schematic for explaining power of reflected beams in the case of transmission (there is no Attenuation). With the use of FIG. 9C, the power of the reflected beam input to output fibers in the case of transmission is explained. FIG. 9D is a schematic for explaining the power of the reflected beams in the case of attenuation. With the use of FIG. 9D, the power of the reflected beam input to the output fibers in the case of attenuation is explained. First, FIG. 9C representing a state (refer to the solid line in FIG. 8C) in which an angle of the reflective surface of the MEMS mirror 4 is set at an angle such that attenuation occurs as little as possible is explained.
In FIG. 9C, a curve 8 represents a power characteristic of a reflected beam 6c reflected at the center portion of the MEMS mirror 4 in FIG. 9B, and a curve 9 represents a power characteristic of the reflected beam 6a reflected at an end of the MEMS mirror 4. As described earlier, reflected beams at the ends of the MEMS mirror 4 have a property to spread spatially. Therefore, it is understood that the curve 8 representing the reflected beam at the center portion has a sharp peak, that the power peak of the reflected beam becomes lower as it deviates from the center portion of the MEMS mirror 4, and that a gentle curve is illustrated in the curve 9 as a whole.
The reason why the peaks of different wavelengths are spatially in the same position is that the reflected beams are not only condensed by the condensing optical system but also translated by angle change by the spectral element 1. Further, the width of a region 7 in FIG. 9C represents an opening up to the input/output optical system 2 composed mainly of the collimator lens 20, and the width becomes wider in accordance with the area of the collimator lens 20 shown in FIG. 8D.
When attention is focused on a relation between the region 7 and each curve, both peaks of the curves 8 and 9 are included in the region 7; however, the peak of the curve 8 is higher, and therefore, the area of the hatched portion under the curve 8 within the region 7 is large and the area of the hatched portion under the curve 9 within the region 7 is small. In other words, the area under the curve 8 is larger than that under the curve 9. This indicates that the power of reflected light to be input to the output fibers becomes lower as it deviates from the center wavelength.
Next, FIG. 9D representing a state (refer to the dotted line in FIG. 8C) in which the angle of the reflective surface of the MEMS mirror 4 is set at an angle such that attenuation occurs is explained. The shapes of the curves are approximately the same as those illustrated in FIG. 9C; however, the region 7 is shifted to left (or right) because the axis is shifted as described earlier.
When attention is focused on a relation between the region 7 and each curve 8 or 9, neither peak of the curves 8 and 9 is included in the region 7. The area of the hatched portion within the region 7 under the curve 8 is small, whereas the area of the hatched portion under the curve 9 within the region 7 is large. In other words, the area under the curve 8 is smaller than that under the curve 9. This indicates that the power of the reflected light reflected at the end rather than the center of the MEMS mirror 4 and deviating from the center wavelength becomes higher.
FIG. 10A is a graph to represent a relation between a band and a transmissivity transmissivity in a case of no influence of diffraction, FIG. 10B is a graph to represent a relation between the band and the transmissivity due to the influence of diffraction, and FIG. 10C is a graph to represent a relation between the band and the transmissivity under the influence of diffraction. In the graphs, the horizontal axis represents the band of frequency (wavelength), and the vertical axis represents the transmissivity. Since the magnitude of cut-off of the reflected beams 6 is increased toward the end surface of the MEMS mirror 4, the influence of diffraction tends to become larger. Therefore, provided that there is no diffraction influence, the change of the beam power results only from the cut-off of the reflected beams 6. Thus, the transmissivity-band characteristic is supposed to indicate a trapezoidal shape in the graph as shown in FIG. 10A. However, the transmissivity-band characteristic indicating a reverse trapezoidal shape as shown in FIG. 10B is added due to the diffraction influence.
Accordingly, a practical transmissivity-band characteristic presents, as shown in FIG. 10C, approximately an M shape indicating that the transmissivity becomes large at the band ends (at both ends) of a certain wavelength. Although the earlier explanation has been described about deviation of the center wavelength, a case in which not only a signal light of the center wavelength but also an amplified spontaneous emission (ASE) light containing wavelength components that deviate from the center wavelength are irradiated on the MEMS mirror 4 is taken into consideration. In such a case of the transmissivity-band characteristic, the characteristic (the transmissivity is small) shows that side lobe portions 12 representing bands where ASE (white noise) generated when an optical amplifier is connected to the optical switch 10 is present have a transmissivity higher than that of a flat portion 11 representing a band where the reflected beams 6 of the signal light exist. This gives rise to a problem that a gain of the ASE becomes larger than that of the signal light when, for example, the optical switch 10 is multistage-connected. Although such a transmissivity-band characteristic is generated even when the angle of the MEMS mirror 4 is changed in any direction, that is, in the horizontal direction or in the vertical direction, there is a difference in degree, and the influence is smaller when the angle of the MEMS mirror 4 is changed in the arrangement direction of the input/output ports (the vertical direction shown in FIG. 8A to 8C).
Next, a difference between the way of beam spreading by diffraction at the ends of the MEMS mirror 4 with respect to the direction perpendicular to the dispersion direction by the spectral element 1 and the way of beam spreading by the diffraction at the ends of the MEMS mirror 4 with respect to the dispersion direction by the spectral element 1 is explained. The diffraction due to the ends of the MEMS mirror 4 with respect to the direction perpendicular to the dispersion direction by the spectral element 1 occurs by a decrease in a maximum vertical diameter of the beam irradiated at the ends of the MEMS mirror 4.
Therefore, the horizontal diameter of the beam having a wavelength whose edge enters slightly the end of the MEMS mirror 4 is decreased; however, its maximum vertical diameter portion is still irradiated on the MEMS mirror 4, and therefore, diffraction is hard to occur. Further, when the end of the MEMS mirror 4 cuts the portion of the maximum vertical diameter, diffraction occurs; however, at this time, over half of the whole beam has already been cut off. Therefore, the power of the light to spread by diffraction becomes smaller, resulting in a curve like the curve 9 shown in FIG. 9C and 9D whose peak is further pushed down.
On the other hand, the diffraction due to the ends of the MEMS mirror 4 with respect to the dispersion direction by the spectral element 1 occurs by a decrease in the maximum horizontal diameter of the beam irradiated to the ends of the MEMS mirror 4. Therefore, as for the wavelength whose edge enters slightly the ends of the MEMS mirror 4, the beam also begins to decrease in the horizontal diameter, and diffraction thus occurs, resulting in spreading of the reflected beam. However, since most of the beam is still irradiated to the MEMS mirror 4, the power of the reflected beam to be diffracted is large.
Accordingly, when the attenuation of the reflected light to be led to the output fibers is carried out by rotating the MEMS mirror 4 in the direction perpendicular to the dispersion direction by the spectral element 1, the peaks of the side lobe portions 12 shown in FIG. 10C become small, and conversely, when the attenuation of the reflected light to be led to the output fibers is carried out by rotating the MEMS mirror 4 in the dispersion direction by the spectral element 1, the peaks of the side lobe portions 12 shown in FIG. 10C become large.
Further, to make the band wide, when the incident beam 5 is made in an oval shape such that the vertical direction is a long axis and the horizontal direction is a short axis on the MEMS mirror 4, most of the beam irradiated to the MEMS mirror 4 is cut off by the time when the diameter in the vertical direction becomes short, and the influence of the diffraction becomes smaller, resulting in that a difference in degree of the diffraction is generated further notably compared with that of the diffraction in the horizontal direction.
FIG. 11 is a graph to represent a simulation example of the band characteristic when an angle is changed to the horizontal direction (wavelength dispersion direction). In FIG. 11, the horizontal axis represents a normalized band, and the vertical axis represents an output transmissivity (decibel (dB)) including insertion loss with respect to input. The variable optical attenuation (VOA) in FIG. 11 is light transmissivity when the angle of the MEMS mirror 4 is variably changed.
When there is no attenuation (VOA=0 dB), the influence of diffraction is not seen, and the band characteristic shows a flat band characteristic in a trapezoidal shape in the graph. As the attenuation becomes larger, the influence of the diffraction is generated. In addition, as shown in FIG. 10C, a characteristic that an output light intensity at the ends of the band rises approximately in an M shape to generate side lobes is seen in the graph (the side lobe portion 12 is referred to as Ear (ear)). For example, when VOA is equal to 10 decibels (VOA=10 dB), the Ear rises by about three decibels with respect to the flat portion.
Thus, when the optical switch that has the band characteristic with the Ear rising higher than the flat portion at the center is used for an optical system, a problem that the protrusions seen in the graph are also amplified at the time of light amplification by an optical amplifier, resulting in deterioration of signal (S)/noise (N) (ratio of signal to noise). This problem becomes particularly notable at the time of multistage connection. Therefore, a system structure having high flexibility could not be conventionally built because the number of multistage connection was limited.